1. Field of the Invention
The present invention is related to the field of electromagnetic induction well logging. More specifically, the present invention is related to methods and apparatus for improving the accuracy of induction well logging instrument by increasing the accuracy with which the phase response of the instrument can be determined and compensated.
2. Description of the Related Art
Electromagnetic induction well logging instrument the used to determine resistivity of earth formations penetrated by wellbores. The induction well logging instrument includes a source of alternating electric current and a transmitter. The transmitter can be a wire coil through which the alternating current is conducted. The alternating current passing through the coil induces corresponding alternating magnetic fields in the earth formations surrounding the wellbore. The induction logging instrument also includes a receiver. The receiver can also be a wire coil, positioned at a spaced apart location from the transmitter. The alternating magnetic fields induced in the earth formation themselves induce alternating eddy current in the earth formation. The magnitude of the eddy currents is related to the conductivity (the inverse of the resistivity) of the earth formations. The eddy current induce alternating voltages in the receiver. Circuits in the induction logging tool measure the magnitude of the voltages induced in the receiver by the eddy current in order to determine the conductivity, and thereby the resistivity, of the earth formation.
A particular difficulty in determining the magnitude of the voltages induced in the receiver by the eddy currents is that the alternating magnetic field from the transmitter itself directly induces voltages in the receiver, these voltages being referred to as direct-coupled voltages. The magnitude of the direct-coupled voltages is typically much greater than the magnitude of the voltages induced by the eddy current. Methods known in the an for reducing the magnitude of the direct-coupled voltages include adding "balancing" coils to the wire coil forming the receiver. The balancing coil is axially positioned relative to the receiver coil, and is series-connected to the receiver coil in inverse polarity, so that a substantial portion of the direct-coupled voltages can be cancelled. In order for the balancing coil to function correctly, it is necessary to build the receiver coil and the balancing coil with a very high degree of geometric precision, and preferably design the coils so as to be highly resistant to changes in geometry and electrical response characteristics as the ambient pressure and temperature to which the coils are exposed changes. Induction well logging instruments are typically exposed to very large variations in pressure and temperature, so design of the receivers and balancing coils is difficult and expensive.
Another technique known in the art for reducing the effect of the direct-coupled voltages is to conduct the output of the receiver to a phase sensitive detector. A phase sensitive detector measures the magnitude of an alternating voltage which is in phase with a phase reference signal. In the induction well logging instrument, the direct-coupled voltages are typically 90 degrees out of phase with the magnetic field created by the alternating current flowing in the transmitter coil. The eddy currents typically lag the transmitter's magnetic field by a 90 degree phase difference. The voltages induced in the receiver coil by the eddy currents lag the eddy currents by a 90 degree phase difference and therefore are typically 180 degrees out of phase with respect to the transmitter's magnetic field. The phase sensitive detector is adapted to measure the magnitude of voltages in the receiver which are exactly 180 degrees out of phase with respect to the current flowing through the transmitter coil. The source of alternating current can be electrically coupled to the phase sensitive detector to provide a phase reference for the phase sensitive detector. However, the phase response of the transmitter itself may not be constant, or may not be precisely known. The phase response of the transmitter affects the degree of time coincidence between the magnitude of the alternating current generated by the source and the magnitude of the magnetic field induced in the earth formation by the current passing through the transmitter coil. If the phase sensitive detector is phase-referenced only to the source of alternating current, then variations in phase response of the transmitter could affect the accuracy with which the phase sensitive detector can determine the magnitude of the eddy current induced voltages in the receiver coil. The direct-coupled voltages induced by the transmitter's magnetic field are typically much larger than the eddy current induced voltages. The direct-coupled voltages may still have a large amplitude relative to the eddy current induced voltages at only a few degrees from 180 degrees phase separation from the transmitter's magnetic field. Small differences in the phase reference, with respect to the phase of the transmitter's magnetic field, when used for the phase sensitive detector can therefore result in large errors in the measurement of the eddy current induced voltages in the receiver.
To overcome the preceding limitation in the induction logging instrument, it is known in the art to provide an additional, low-gain antenna (which can be a wire coil) near the transmitter to measure directly the magnitude of the magnetic field generated by the transmitter. The transmitter's magnetic field induces voltages directly into the additional antenna. The voltages induced directly in the low-gain antenna are substantially time correspondent with the magnetic field generated by the transmitter current. The voltages induced in the low-gain antenna can be used as a phase reference for the phase sensitive detector when shifted by 90 degrees.
A limitation to using the low-gain antenna is that the phase response of the receiver coil, and analog amplifiers which are typically connected to the receiver coil, may not be precisely known. The receiver coil may have variations in phase response to voltage inducted by the eddy currents. Variation in phase response can cause the eddy current induced voltage in the receiver coil not to be always precisely 90 degrees out of phase with the eddy current magnitude. Analog amplifiers are typically connected to the receiver coil to increase the magnitude of the voltages induced in the receiver coil to a level compatible with the signal input range of the phase sensitive detector or an analog-to-digital converter. If the phase sensitive detector is referenced only to the transmitter's magnetic field, then variations in the combined phase response of the receiver and the analog amplifier can result in error in determining the magnitude of the voltages induced in the receiver by the eddy current.
U. S. Pat. No. 4,720,681 issued to Sinclair describes a system for calibrating the phase response of the analog amplifier connected to the receiver coil (the "receiver amplifier"). The system described in the Sinclair '681 patent includes a switch for momentarily conducting a portion of the current supplied to the transmitter to the input of the receiver amplifier. Phase response of the receiver amplifier can be compared to the transmitter current reference to calibrate the receiver amplifier phase response.
A drawback to the system disclosed in the Sinclair '681 patent is that it only calibrates the receiver amplifier. The receiver coil is switched out of the receiver circuit during calibration. The phase response of the receiver coil itself remains uncalibrated. The phase response of the receiver coil becomes increasingly important at higher operating frequencies contemplated by more modern induction logging tools. Another drawback to the system disclosed in the Sinclair '681 patent is that is uses the transmitter current for a phase reference. At low alternating current frequencies, typically less than about 40 KHz, the phase response of the transmitter has an insignificant effect on the accuracy with which the alternating current may be used as a phase reference for the phase sensitive detector. At higher alternating current frequencies, the phase response of the transmitter can have a significant effect on the accuracy of the alternating current as a phase reference.
Accordingly, it is an object of the present invention to provide an improved system and method for determining and compensating for variations in the phase response of the transmitter, the receiver coil and analog amplifier in an induction well logging instrument.